/Engine/Source/Runtime/Slate/Public/Widgets/Layout/SSplitter.h
Given a mouse position within the splitter, figure out which resize handle we are hovering (if any).
The index of the handle being hovered, or INDEX_NONE if we are not hovering a handle.
template<EOrientation SplitterOrientation> static int32 GetHandleBeingResizedFromMousePosition ( float PhysicalSplitterHandleSize, float HitDetectionSplitterHandleSize, FVector2D LocalMousePos, const [TArray](API\Runtime\Core\Containers\TArray)< [FLayoutGeometry](API\Runtime\SlateCore\Layout\FLayoutGeometry) > & ChildGeometries )